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Moisture MetricsRed Calc Tool User Guide

What this tool can do for you

You can calculate up to seven moisture metrics -- saturation vapor pressure,
relative humidity, dew point temperature, water vapor density, vapor
pressure, wet-bulb temperature, and and humidity ratio -- merely by entering
dry-bulb temperature and one of the previous values. It's like having all
the benefits of a psychrometric chart without the chart complexities.

This unique RED Calc tool uses our "solve-all" technique, but in a different
way than most of our other solve-all tools. Instead of selecting one of six values
to solve, you select one of six to enter; the other five are
solved by the tool. This tool works most similarly to our Air Leakage
Metrics tool.

Sling psychrometer with wet-bulb (top)
and dry-bulb thermometers

Calculated values

Saturation vapor pressure - at a given temperature, the pressure
at which water vapor and liquid water can exist in equilibrium.
This is also known as the equilibrium vapor pressure.

Relative humidity - the ratio of the density of the water vapor to
the density at saturation, at the same dry-bulb temperature.
Equivalently, this is the ratio of the current vapor pressure to
the saturation vapor pressure.

Dew point temperature - the temperature at which water vapor reaches
its saturation point, or 100 percent relative humidity. Below this
temperature condensation begins to occur.

Water vapor density - the density of water vapor.

Vapor pressure - the pressure exerted by water vapor. If mixed
with air, this is the partial pressure of the water vapor
(fraction of air pressure, by number of molecules).

Wet-bulb temperature - the temperature indicted by a thermometer with a sensing
element covered by a water-saturated wick over which air is
caused to flow at a rate of at least 500 feet per minute (2.54 m/s).
This thermometer and wick combination is called a wet-bulb
thermometer. The heat for vaporization from the wick is supplied by the
surrounding air.

Your body is sensing the wet-bulb temperature as you get out of
the shower, but before you dry off. Your skin is
supplying the thermal energy for the evaporation of the liquid water.
Once you dry off, the energy for evaportation is no longer needed,
so you feel warmer.

Humidity ratio - the ratio of the mass of the water vapor to the mass of dry air
for a given volume.

Tips

Clicking the label for any input or result will cause a
popup help box to appear. This help box includes the
allowed values (for inputs).
Read more.

For an enhanced conceptual understanding of moisture metrics,
use a psychrometric chart in combination with this Moisture Metrics tool.
There is a psychrometric chart below in the Background section.

The relationship between the three temperatures involved is
Dew point ≤ Wet-bulb ≤ Dry-bulb.
At saturation (100 percent relative humidity) all three temperatures are the same.

All temperatures in this tool (dry-bulb, wet-bulb, and dew point)
must fall between -15 °C (5 °F) and
374 °C (705 °F) because the calculations depend upon
laboratory measurements that are only valid in this range. This imposes
restrictions (directly or indirectly) on the allowed ranges of all the
inputs.

The allowed ranges of the inputs in the bottom section of the tool depend
on the given Dry bulb temperature.

The Humidity ratio (the last result) of this RED Moisture Metrics tool
is a ratio without mathematical units. If you are accustomed to working
with this value in units of grains of water vapor per pound of dry air,
merely multiply the Humidity ratio by 7000 (there are 7000 grains in one
pound). For example, to find grains of water vapor per pound of dry air
for a Humidity ratio of 0.008: 7000 grains/pound X 0.008 = 56 grains of
water vapor per pound of dry air.

Mold growth is supported when the relative humidity is above 60
percent.

Inputs and field measurements

Dry-bulb temperature - temperature of air measured with an ordinary
thermometer (dry-bulb thermometer).

One of the following six metrics in the bottom section of the tool
must be entered as an
input. Click the radio button to select the metric you wish to enter;
the five other moisture metrics will be calculated.

Relative humidity - see "Calculated values" above.

Dew point temperature - see "Calculated values" above.

Water vapor density - see "Calculated values" above.

Vapor pressure - see "Calculated values" above.

Wet-bulb temperature - see "Calculated values" above.

Humidity ratio - see "Calculated values" above.

Practical applications

As a sample or mass of air cools, its relative humidity increases.
If it continues to cool, its relative humidity will eventually reach 100 percent,
the dew point temperature. If it cools below the dew point (saturation)
temperature, some of the water vapor in the air will change phase to liquid
water. This is what might occur when moisture-laden air moves through a wall.

Condensation of water vapor often takes place on basement walls with an
interior surface temperature below the dew point temperature of the
basement air. This can lead to hazardous mold and mildew in a basement.
It is common for homeowners to open basement windows if they discover such
condensation. In many cases, this only makes the condition worse because
the outdoor air is the major source of moisture in the basement. The basement
windows should only be opened when the dew point temperature of the outdoor
air is lower than the interior temperature of the basement wall.

In order
to estimate the interior temperature of an uninsulated basement wall below grade,
subtract the latitude of the house from 90 degrees. For example,
90 degrees minus 40 degrees north latitude = 50 degrees Fahrenheit.

Water vapor diffusion through a material is driven by a difference
of water vapor pressure across the material. You can use this tool
to calculate the vapor pressure on both sides of the material and
use that to determine the direction of water vapor diffusion. The
vapor will move from higher vapor pressure to lower vapor pressure.
By multiplying the difference in vapor pressure by the permeability of
the material, you can calculate the rate of diffusion.

Background

This Moisture Metrics tool solves most of the values on a typical
psychrometrics chart (see below), including relative humidity, dew point
temperature, wet-bulb temperature, and humidity ratio.

As you can see from the inputs and results of the Moisute Metrics tool, it has to do with
the properties of air/water-vapor mixtures. Heating, ventilation, and air conditioning
engineers use psychrometrics to determine values such as the energy required
to remove water vapor from air, or to determine at what temperature water
vapor will condense on a cool surface. The folks who predict weather use
psychrometrics to determine when it is going to rain or snow.

For building science professionals, psychrometics has many uses. These
include the prediction of condensation on cool surfaces;
explaining why running ventilation during cold weather has a drying
effect in the indoor air; understanding why objects made of wood dry out
if they are near a hot appliance, such as a wood stove; and comprehending
the need for air and vapor barriers in dwelling enclosures.

Best practices

During cold weather when a dwelling is heated, it is recommended the
relative humidity be kept within a range of 30 to 50 percent
(a humidity ratio of 0.0046 to 0.0078 at 70°F (21°C)).

A sling psychrometer is probably the best tool for measuring the moisture
characteristics of a sample of air. This instrument includes a dry-bulb and
wet-bulb thermometer mounted on a hinged device that allows the operator to
sling it though the sample of air for at least 90 seconds at a rate of at
least 500 feet per minute (2.54 meters per second). Please see the photograph
at the beginning of this User Guide.

Lstiburek, J. and Carmody, J. Moisture Control Handbook: Principles and
Practices for Residential and Small Commercial Buildings. Wiley and Sons, New York, NY. 1994.Comment: Thorough text with helpful illustrations that enhances
understanding of moisture control for building scientists.